Any electoral system has an electoral formula that converts vote
proportions into parliamentary seats. Pre-electoral polls usually focus
on estimating vote proportions and then applying the electoral formula
to give a forecast of the parliament's composition. We here describe the
problems arising from this approach: there is always a bias in the
forecast. We study the origin of the bias and some methods to evaluate
and to reduce it. We propose some rules to compute the sample size
required for a given forecast accuracy. We show by Monte Carlo simulation
the performance of the proposed methods using data from Spanish elections
in last years. We also propose graphical methods to visualize how electoral
formulae and parliamentary forecasts work (or fail).